Sonographic evaluation of median nerve cross‐sectional area in a normal Iranian population: A cross‐sectional study

Abstract Introduction Considering disagreements on the normal range of median nerve cross‐sectional area (MNCSA) and insufficient data in the Iranian population, this study aimed to measure normal MNCSA. Methods In this cross‐sectional study, bilateral upper limbs of 99 subjects were assessed by sonography, and MNCSA was measured at three levels: forearm, carpal tunnel inlet (CTI), and carpal tunnel outlet (CTO). The association between MNCSA and demographic factors was assessed. Results Mean MNCSA was 6.33 mm2 at the forearm, 9.41 mm2 at CTI, and 10.67 mm2 at CTO. MNCSA was significantly higher in males (6.78 vs. 5.94 mm2 at the forearm, 9.98 vs. 8.92 mm2 at CTI, and 11.24 vs. 10.84 mm2 at CTO in males and females, respectively) and taller (>170 cm) subjects in all three levels (6.69 vs. 6.03 mm2 at the forearm, 9.80 vs. 9.02 mm2 at CTI, and 11.27 vs. 10.12 mm2 at CTO in taller and shorter subjects, respectively). MNCSA was not significantly associated with wrist ratio (WR) or body mass index (BMI). Conclusion The normal MNCSA range in the Iranian population is 6.31 mm2 (forearm) to 10.74 mm2 (CTO). MNCSA is significantly higher in males and taller subjects but is not associated with BMI and WR.

sonographic criteria have not been fully determined for CTS, some sonographic findings favor this diagnosis.
Among the several indicators, increased median nerve crosssectional area (MNCSA) is the most significant and possibly most reliable finding. 3 MNCSA can be measured in different locations, including the forearm, the carpal tunnel inlet (CTI) (at the level of pisiform), and the carpal tunnel outlet (CTO). Among these, MNCSA at the CTI level is more frequently studied and is increased in CTS. A study comparing sonography with electrodiagnosis demonstrated that both methods have the same sensitivity for diagnosing CTS. 4 However, another study compared the results of MNCSA measurement with electrodiagnosis and found that sonography is a fairly sensitive method for diagnosing CTS but still not competent enough to replace electrodiagnosis. 5 There are several studies with different results for a normal range of MNCSA. In the most recent review, the mean MNCSA was 8.81 mm 2 at mid-arm, 8.57 mm 2 at the elbow, 7.07 mm 2 at midforearm, 8.74 mm 2 at CTI, and 9.02 mm 2 at CTO. 6 Another study reported normal MNCSAs to be 8.2 mm 2 at the level of the radiocarpal joint, 8.3 mm 2 at CTI, and 8.1 mm 2 at CTO. 7 A handful of studies have provided reference values for MNCSA in certain sample populations. 8,9 However, such studies have yet to be widely used. Furthermore, they are often limited to a specific population and may not be useful for all future research.
Although studies on normal range of MNCSA have demonstrated differences among various populations, to the best of our knowledge, it has not yet been studied in the Iranian population. This study was designed to measure normal MNCSA in the Iranian population, as and to evaluate its association with sex, body mass index (BMI), wrist ratio (WR), and height.

| RESULTS
The data for 198 hands from 99 participants (46 men and 53 women) were analyzed. The mean age of subjects was 41 years . The mean WR was 0.67, and the mean BMI was 25.6 ( Table 1). The mean MNCSA was 6.33 mm 2 in the forearm (ranging from 3.0 to 9.8 mm 2 ), 9.41 mm 2 in CTI (ranging from 4.4 to 14.0 mm 2 ), and 10.67 mm 2 in CTO (ranging from 5.3 to 16.2 mm 2 ; Table 2).
Sex distribution included 53 females and 46 males. MNCSA was significantly higher in men (6.78 vs. 5.94 mm 2 at the forearm, 9.98 vs. 8.92 mm 2 at CTI, and 11.24 vs. 10.84 mm 2 at CTO for males and females, respectively; Table 3).
The mean MNCSA was significantly different between height groups and at all levels. The results showed that taller subjects had a larger MNCSA (6.69 vs. 6.03 mm 2 at the forearm, 9.80 vs. 9.02 mm 2 at CTI, and 11.27 vs. 10.12 mm 2 at CTO for >170 cm and ≤170 cm subjects, respectively; Table 3).
WR was measured in all subjects, and mean values were 0.68 among males and 0.66 among females, which was not significantly different. Also, no significant difference was observed between the MNCSA of the WR groups (6.45 vs. 6.23 mm 2 at the forearm, 9.52 vs. 9.33 mm 2 at CTI, and 10.88 vs. 10.50 mm 2 at CTO for WR >0.7 and ≤0.7, respectively; Table 3).
The mean BMI was 26.5 for men and 24.8 for women. No significant difference was observed in the MNCSA of the two BMI groups (6.37 vs. 6.11 mm 2 at the forearm, 9.58 vs. 9.25 mm 2 at CTI, and 10.84 vs. 10.50 mm 2 at CTO for BMIs >25 and ≤25, respectively; Table 3).
The measurements had good intra-observer reliability (Table 4).

| DISCUSSION
The clinical significance of measuring MNCSA is in detecting CTS.
Inflammation and secondary edema happen after an increased pressure in the canal, which leads to nerve enlargement, which is  11,12 In this cross-sectional descriptive study on 99 healthy participants, bilateral MNCSA was measured at three levels: forearm, CTI, and CTO. MNCSA ranged from 6.31 mm 2 at the forearm to 10.74 mm 2 at CTO. Although normal upper limits were not measured, the present study found mean values to be 9.41 mm 2 at CTI and 10.67 mm 2 at CTO. Kang et al. determined forearm MNCSA to be 6.8 mm 2 , which is very close to the findings of this study. 13 In another study, the mean value of wrist MNCSA was estimated to be 8.5-11 mm 2 , which is also close to this study's findings. 14  4.8 mm 2 at the mid-forearm level, which is significantly smaller than the findings of this study. 15 The variety in normal values can be due to differences in age, weight, height, sex, and ethnicity. [16][17][18][19] Regarding ethnicity, Ng  be significantly higher in males. 21 In a study on CSA of the sciatic nerve, women had smaller CSA than men. 18

CONFLICT OF INTEREST STATEMENT
The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT
The data that support the findings of this study are available from the corresponding author upon reasonable request.

TRANSPARENCY STATEMENT
The lead author Masume Bayat affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.